The reinforcement of a reinforced-concrete structure to enhance the strength of the structure usually consists of longitudinal principal bars and reinforcing forcing steel in the form of stirrups or hoops wound around the longitudinal principal bars. The reinforcing steel serves to prevent rupture of the concrete and buckling of the longitudinal principal bars due to the longitudinal principal bars being displaced in a direction normal to the axis thereof by an axial compressive force.
In the arrangement of the reinforcing steel work, the reinforcing bars are conventionally wound around the longitudinal principal bars as a stirrup or hoop as follows: first length of reinforcing bar is cut for matching the periphery of the longitudinal bars to be strengthened. The cut lengths are manually wound one after another around the longitudinal principal bars and they are fixed to the latter by binding. By repeating this process a number of stirrups or hoops are bound at specified intervals, and the steel work is finally anchored as a reinforcement in the concrete structure. Thus the stirrups or hoops are each an independent bar.
This method of manually winding one piece after another as the stirrup or hoop around the longitudinal principal bars to form the steel work is an extremely primitive and inefficient one. Besides, in a reinforced-concrete column for instance, when the column is under loading the longitudinal principal bars and the ambient concrete tend to be displaced in a lateral direction by an axial compression or seismic force, the loop portion of the hoop steel tends to slip out of the concrete, and the restraint for the longitudial principal bars and concrete provided by the hoop steel fails and in consequence a buckling of the longitudial principal bars leads to rupture of the concrete.
To avoid such problems, use of a helical bar for reinforcing has been proposed. Namely a long bar of the same quality material as a conventional reinforcing bar is bent so as to form an identical loop continuously or coiled and successively folded to form a continuous spiral. Use of a helical bar for reinforcing eliminates the step of manually winding one piece after another of the reinforcing bar around the longitudinal principal bars and fixing them thereto. The construction of the steel reinforcing becomes that much more efficient.
In the proposed method however, it is difficult in the field to make sure that the reinforcing steel hoop is fixed with a specified pitch at a specified position around the principal bars because the spiral steel in this case is a reinforcing bar of the same quality material as the conventional reinforcement but is in a folded state. Therefore when it is stretched out, it develops plastic deformation and it requires considerable time and labour to stretch it to a desired uniform pitch around the principal bars. For instance, to construct a steel cage 3m high, 700mm square, having coils at a 200mm pitch each of which is a square hoop 13mm in diameter as a conventional reinforcement for a column, the spiral or coil is compressed and then stretched again, and as a result a residual deformation is observed. Such a spiral or coil weighs as much as 44kg and it is a tremendous job to coil such a heavy bar around the principal bars while on a high scafolding and while making a pitch correction. In the present practice, such a spiral bar which is as long as 3mm is divided into several parts to reduce the weight which must be handled, and sometimes a special hanger must be provided. As a result, poor workability, low efficiency and poor economy are unavoidable.
Moreover, as mentioned above, in a concrete structure with a steel cage buried therein the axial compressive force can be resisted by the concrete, while the tensile force which can be withstood is governed by the principal bars; and the main role of the reinforcing hoops is to strengthen the longitudinal principal bars, and speaking in terms of architectural dynamics or structural dynamics it cannot be said that due attention has been paid to resistance to force in a direction normal to the bar axis or the shearing force. This is a very real problem when the concrete structure is a structural member of an aseismic building.